Historically, the mitral valve is described as composed of the leaflets, chordae, and papillary muscles. However, the mitral valve structurally and functionally is part of the left ventricle and intimately associated with the atrium and fibrous skeleton of the heart. Thus, alterations to the fibrous skeleton and ventricular and atrial muscle contribute to and affect valvular function.
The leaflets are the valve component that creates the division between the atrium and ventricle. There are two distinct leaflets: the anterior or aortic and posterior or mural. The anterior leaflet is usually comprised of a single trapezoidal-shaped unit. The posterior leaflet is punctuated with multiple slits and clefts that define usually three, but up to six, distinct scallops (Fig. 2.1)
(though often not depicted this way in line drawings in texts). The anterior and posterior leaflets are separated at the commissures but there is usually some continuity of the valve tissue close to the annulus. Further, if one looks at the chordal distribution, at the commissures, chordae are distributed to both leaflets from a common structural source—so the leaflets form a single functional unit. Nevertheless, it is useful for the purposes of valve repair to differentiate anterior from posterior. However, one must remember that multiple separately suspended distinct units come together to make the valve competent. To describe components of mitral valve repair it is useful to label the scallops by position (Fig. 2.2); posteriorly from left to right P1, P2, P3, with corresponding regions of the anterior leaflet A1, A2, and A3 (1,2).
Figure 2.1 Photograph of the mitral valve. Note multiple clefts in the posterior leaflet (arrows). (From Angelini A, Ho SY, Thiene G, Anderson RH. Anatomy of the mitral valve. In: Boudoulas H, Wooley CF, eds. Mitral Valve: Floppy Mitral Valve, Mitral Valve Prolapse, Mitral Valvular Regurgitation. 2nd ed. Armonk: Futura Publishers; 2000:9.)
Figure 2.2 Segments of the mitral valve. (From Kouchoukos NT, Blackstone EH, Doty DB, Hanley FL, Karp RB. Kirklin/Barratt-Boyes Cardiac Surgery: Morphology, Diagnostic Criteria, Natural History, Techniques, Results, and Indications. Philadelphia: Churchill Livingstone; 2003:21.)
When in the closed position, the orifice is obscured by the anterior and posterior leaflets. The anterior comprises about one half to two thirds of this area, the posterior comprises about one third to one half of this area. The point of attachment of the anterior leaflet comprises one third of the circumference of the annulus (the fibrous area) and the posterior leaflets comprises two thirds of the circumference of the annulus (the muscular area). The fibrous support of the anterior leaflet is fixed, the muscular support of the posterior leaflet can enlarge and does so when annular dilatation is associated with mitral regurgitation. The leaflets themselves are comprised of clear and rough zones (Fig. 2.3). The clear zone is between the line of closure and the annulus and can be quite thin, almost transparent. The rough zone extends from the line of closure to the free edge, characterized by thicker, nodular ridges which promote sealing of the orifice on valve closure (3).
Figure 2.3 Zones of the mitral leaflets. (From Seccombe JF, Schaff HV. Mitral valve repair: current techniques and indications. In: Franco KL, Verrier ED, eds. Advanced Therapy in Cardiac Surgery. St. Louis: B.C. Decker; 1999:222.)
Figure 2.4 Relational anatomy of the mitral valve. LFT and RFT, left and right fibrous trigones; AL and PL, anterior and posterior mitral leaflets; PV, AV, and TV, pulmonic, aortic, and tricuspid valves. (From Chitwood Jr. WR. Mitral valve repair: ischemic. In: Kaiser LR, Kron IL, Spray TL. Mastery of Cardiothoracic Surgery. Philadelphia: Lippincott-Raven Publishers; 1998:310.)
Of the four heart valves, the mitral valve is the only valve that has a distinguishable annulus. However, the presence of a fibrous annular structure is variable and discontinuous. The firmest site of support for the mitral valve is the region of fibrous continuity between the aortic and mitral valves, the extent of which is delineated by the right and left fibrous trigones (Fig. 2.4). Nevertheless, for surgical purposes, the annulus is considered the area of attachment of the valve leaflets to the atrial muscle. The annulus is a functional component of the mitral valve. The annulus is quite flexible and changes shape throughout the cardiac cycle (4). With normal systolic function, the annulus will reduce in size by 20% to 40%.
Functionally the mitral annulus is not two-dimensional but in fact three-dimensional assuming a saddle shape (Fig. 2.5) (5). The curvature imposed by the saddle shape reduces mechanical stress on the leaflets. The curvature of the saddle shape or height of the “saddle horn” is reduced in a model of ischemic mitral regurgitation (6). This may have implications in choosing techniques for annular reduction and stabilization.
The tendinous chords, as functional extensions of the papillary muscles, perform the dual function of maintaining valvular competence by preventing leaflet prolapse and maintaining ventricular geometry by providing cross-ventricular support. The chords originate from the apical portions of the papillary muscles or directly from the posterior ventricular wall. They insert either into the free edge of the leaflets or on their ventricular surface. Chords
have been characterized a number of ways. The most useful for repair purposes is to describe the first-degree chords as those inserting into the valve edge and the second-degree chords as those inserting into the underside of the leaflet. Both first-degree and second-degree chords originate from the papillary muscles and third-degree chords originate from the ventricular wall and insert into the base of the posterior leaflet.
Figure 2.5 Shape of the human mitral annulus. Fourier smoothed reconstruction from images obtained by trans-esophageal echocardiography. The commissures are located at the low points of the saddle. (From Salgo IS, Gorman JH 3rd, Gorman RC, et al. Effect of annular shape on leaflet curvature in reducing mitral leaflet stress. Circulation. 2002; 106:711-717.)
There are two papillary muscles associated with the mitral valve. When looking from the atrial side, the anterolateral is to the left and the posteromedial is to the right. The muscles are located under their respective commissures. They originate from the lower third of the left ventricular free wall. The papillary muscles are extensions of the ventricular muscle from which they originate. The chords originate from the fibrous tips of these muscles, extending to insert on the mitral leaflets. The blood supply of the anterolateral papillary muscle derives from the left circumflex and/or diagonal systems. The blood supply of the posteromedial papillary muscle derives from the posterolateral coronary branches whether it originates from the left or right coronary system. The papillary muscles adjust tension and stabilize the valve during the cardiac cycle, rather than pulling the chords and leaflets into position. The function of the papillary muscles and their position is greatly dependent on the status of the ventricular myocardium underlying it. Displacement of the papillary muscle by distorted ventricular geometry is one mechanism for the creation of mitral regurgitation.